In the prior art it is well known to solder, fuse or braze articles by condensing hot saturated vapor of a heat transfer liquid thereon. Typically such operations are performed in a chamber having a reservoir of the heat transfer liquid therein. Immersion heating coils in the reservoir are activated to boil the liquid while cooling coils mounted in the upper portion of the chamber are used to condense vapor from the boiling liquid. Such an arrangement forms a confined body of hot saturated vapor by substantially precluding the escape of the vapor from the chamber. The articles are then immersed in the body of vapor which condenses thereon and gives up its latent heat of vaporization thereto to heat the articles to the temperature required for soldering, brazing or fusing. Such a technique is described in U.S. Pat. No. Re. 30,399 which issued on Sept. 9, 1980 and is assigned to Western Electric Company and Bell Telephone Laboratories, Inc. and is incorporated by reference herein.
This technique has proven to be eminently successful, however, solder flux placed on the article to be soldered is washed therefrom by the liquid condensate and falls into the reservoir of the boiling heat transfer liquid. The flux and other contaminants coming in contact with the immersion heaters can cause "hot spots" on the heaters which can lead to thermal degradation of the heat transfer fluid. Also, the presence of large quantities of flux in the heat transfer fluid accelerates the formation of acid within the vapor environment which leaves undesirable ionic contamination on processed parts and shortens the useful life of the chamber. Additionally, the heat transfer liquid is expensive necessitating repeated reuse from an economic standpoint.
U.S. Pat. No. 4,077,467 describes a system for removing the flux from the heat transfer liquid. The mixture of the high temperature (e.g., 419.degree. F.) liquid with the flux suspended therein is drained from a reservoir into a cooling chamber, the flux thereby becoming solidified in suspension. The suspended, solidified flux is then filtered out by a mechanical filter connected to the output line of the cooling chamber. The heat transfer liquid passing through the mechanical filter is then pumped back into the reservoir.
The mechanical filters are comprised of a housing having a replaceable fiber mesh cartridge therein. The fiber being one of a variety of yarns or synthetic materials depending on the constituents of the flux and/or the temperature of the liquid. Such mechanical filters have been found to quickly clog up and must be monitored often and cartridges changed at least daily depending upon the particular product being soldered and the amount of flux used.
Accordingly, there exists a need for a reuseable filter which is effective for removing flux from a liquid-flux mixture and which requires a minimum of maintenance and/or cleaning.